After you have extended the logical volume it is necessary to increase the file system size to match. how you do this depends on the file system you are using.

By default, most file system resizing tools will increase the size of the file system to be the size of the underlying logical volume so you don't need to worry about specifying the same size for each of the two commands.

1.

ext2/ext3

Unless you have patched your kernel with the ext2online patch it is necessary to unmount the file system before resizing it. (It seems that the online resizing patch is rather dangerous, so use at your own risk)

I have recently expanded my RAID successfully as well and wrote a wiki article. I tried to take a "safe" approach and have the RAID unmounted in single-user mode to reduce the risk of something going wrong.